The invention relates to a molding composition for the production of moldings whose surface has a particular, decorative effect and which are used, in particular, in the interior of motor vehicles.
Interior trim for motor vehicles, such as covers for A, B and C columns and the like, are currently usually produced from thermoplastics by injection molding. The preferred material recently is reinforced, bulk-dyed polypropylene. For visual reasons, these parts are provided with a surface structure. These structures are generated by corresponding engraving in the injection molds. The result is moldings having a structured, colored, but homogeneous surface appearance.
In order to satisfy increased demand with respect to appearance and comfort, the trim can be painted or laminated with films or textile materials. The lamination is in each case carried out with the aid of adhesives. Depending on the nature of the support material, pretreatment of the moldings by flame treatment, corona discharge or the like is necessary in order to achieve good adhesion. These individual process steps mean relatively high costs. A further, associated disadvantage is the unfavorable fogging behavior.
Fogging is caused by condensation of (evaporated) volatile interior fittings of the motor vehicle on the windows, in particular on the windscreen. The fogging intensity is naturally dependent on a number of factors and on the prevailing temperature conditions. Since adhesives frequently contain a relatively high proportion of volatile substances, adhesive-laminated parts virtually always represent a significant fogging-promoting potential.
A further disadvantage can occur in adhesive-laminated parts with respect to design freedom. Due to the restricted deformability of textiles and films during the lamination operation, limitations may have to be taken into account with respect to the geometrical design of the support parts.
The object was therefore to find a material which, when injection molded, gives moldings which have a textile-like appearance.
It is in principle possible for colored PP, if desired additionally modified by means of reinforcing materials, such as glass fibers or talc and/or elastomers, to be provided with fibers of a different color. Thermally and mechanically highly resistant fibers of, for example thermoplastic polyester, polyamide or polyacrylonitrile can be admixed with the polypropylene in an amount of from about 0.5 to 3% and homogeneously mixed with the polymer in extruders. Test sheets produced therefrom and provided with a grained surface had a textile-like appearance. However, attempts to produce large-area moldings in which geometrically required long flow paths are present had proceeded unsatisfactorily. The reason was inadequate thermal and mechanical resistance of the polyester, polyamide or polyacrylonitrile fibers. The processing temperature of from about 230 to 270xc2x0 C. necessary during injection molding of polypropylene and the material shearing occurring in screw injection molding machines during homogenization and during flow through hot channels and narrow gates resulted in considerable thermal damage to the fibers. Furthermore, these shear forces, in combination with the high temperature, in some cases even caused degradation of the fiber geometry.
Organic fibers, such as wool or cotton, proved to be more resistant with respect to shearing. However, discoloration occurred at the conventional processing temperature, even after a short residence time in the injection-molding machine, as a consequence of thermal load, resulting in a brown coloration and partial decomposition during production interruptions.
It has now been found that the object can be achieved by use of carbon fibers.
The use of carbon fibers to reinforce polymers is known per se. Usually, amounts of from 10 to 20% of cut carbon fibers are added to polymers in order to achieve a significant improvement in the mechanical properties (flexural strength, etc).
The invention thus relates to a polypropylene molding composition for the production of moldings having a decorative effect, containing a polymer of propylene and from 0.3 to 3% by weight, based on the molding composition, of carbon fibers having a fiber length of from 0.5 to 18 mm.
The base material for the molding composition according to the invention comprises
a) from 95 to 50% by weight, preferably from 90 to 70% by weight, of isotactic polypropylene or copolymers of propylene containing up to 15% by weight of ethylene,
b) from 5 to 50% by weight, preferably from 10 to 30% by weight, of rubber-like copolymers which are compatible with polypropylene, and
c) from 10 to 50% by weight, preferably from 10 to 40% by weight, of reinforcing fillers.
Preference is given to a homopolymer or copolymer of propylene having a melt flow index MFI 230/5 in accordance with DIN 53 735 of from 1 to 95 g/10 min.
Suitable rubber-like copolymers are, in particular, amorphous copolymers comprising from 30 to 70% by weight of ethylene and from 70 to 30% by weight of propylene, terpolymers of ethylene, propylene and up to 5% by weight of dienes, preferably ethylidenenorbornene or 1,4-hexadiene, copolymers of ethylene and from 10 to 45% by weight of vinylacetate, or block copolymers of styrene and butadiene or styrene and isoprene, with polystyrene blocks at both ends of the molecule.
The reinforcing fillers employed are preferably talc, chalk, glass fibers or glass beads.
The decorative material employed is carbon fibers having a length of 0.5 to 18 mm, preferably 1 to 6 mm. The amount added is from 0.5 to 3%, preferably from 1 to 2%, based on the total molding composition.
The molding composition according to the invention may furthermore contain the conventional additives which simplify processing and improve the physical properties. Examples which may be mentioned are light and heat stabilizers, antioxidants, antistatics, lubricants, and colored pigments and flameproofing agents. The first group is generally present in the molding composition in an amount of from 0.01 to 5% by weight, calculated on the amount of polymer (plus filler). Fillers, colored pigments and flameproofing agents are employed in an amount corresponding to requirements.
The processing conditions for the molding composition according to the invention correspond to the conditions normally used in the processing of polypropylene. The processing temperatures, measured immediately after the composition leaves the nozzle, are in the range from 240 to 280xc2x0 C., depending on the size and complexity of the molding. The mold temperature is generally from 40 to 70xc2x0 C.
For the production of particularly large-area moldings which are difficult to process, particularly high processing temperatures may also be selected for these articles without impairment of color or properties taking place.
An optimum effect is obtained when pale-colored, preferably white, gray, blue or brown, polymers are used. This effect can also be influenced by surface treatment of the moldings.
Suitable processing methods are, for example, injection molding, extrusion and extrusion blow molding.